Abstract: In order to study the swirl flow effects on the performance of axisymmetric nozzle,the flow field in axisymmetric nozzle was numerically simulated under both swirl flow and straight flow conditions.The flow coefficient,vacuum thrust coefficient and specific impulse of the nozzle were studied with different air mass flow rates and swirl intensities.It is concluded that the flow coefficient,vacuum thrust coefficient and specific impulse are not changed under different swirl intensities if calculated from the axial mass average total pressure at the nozzle inlet.The tangential dynamic pressure increase at the nozzle inlet with the swirl number enlarged at the combustion chamber inlet,and do not contribute to the thrust,so it should be looked as another type of total pressure loss.